The present situation index declined to 39.3 from 40.2, and the index for future expectations declined to 75.2 from 83.2 last month.
The cutoff date for these preliminary results for the month was May 18, 2011.
Lynn Franco, Director of The Conference Board Consumer Research Center commented: "A more pessimistic outlook is the primary reason for this month's decline in consumer confidence.
The Conference Board's consumer confidence index decreased to 60.8 in May, down from 66.0 in April. Economists had expected an increase to 67.5 for May.